Well Test Burner System and Method

ABSTRACT

Methods and systems of burning waste effluent include controlling flow of waste effluent to burner nozzles using flow control elements. The flow control elements are biased toward closed positions using biasing forces, wherein reduced biasing forces are applied to selected flow control elements so that waste effluent flow first reaches the burner nozzles associated with the flow control elements receiving reduced biasing forces. The selected burner nozzles may be located nearer a pilot to more reliably induce combustion and reduce fallout.

BACKGROUND OF THE DISCLOSURE

Hydrocarbons are widely used as a primary source of energy, and have a significant impact on the world economy. Consequently, the discovery and efficient production of hydrocarbon resources is increasingly important. As relatively accessible hydrocarbon deposits are depleted, hydrocarbon prospecting and production has expanded to new regions that may be more difficult to reach and/or may pose new technological challenges. During typical operations, a borehole is drilled into the earth, whether on land or below the sea, to reach a reservoir containing hydrocarbons. Such hydrocarbons are typically in the form of oil, gas, or mixtures thereof which may then be brought to the surface through the borehole.

Well testing is often performed to help evaluate the possible production value of a reservoir. During well testing, a test well is drilled to produce a test flow of fluid from the reservoir. During the test flow, key parameters such as fluid pressure and fluid flow rate are monitored over a period of time. The response of those parameters may be determined during various types of well tests, such as pressure drawdown, interference, reservoir limit tests, and other tests generally known by those skilled in the art. The data collected during well testing may be used to assess the economic viability of the reservoir. The costs associated with performing the testing operations may be significant, however, and therefore testing operations should be performed as efficiently and economically as possible.

Fluids produced from the test well are generally considered to be waste and therefore are typically disposed of by burning, which raises environmental and safety concerns. Conventional burners may be configured to atomize the waste effluent prior to combustion. The waste effluent, however, typically must be provided at a minimum pressure for effective atomization. When the waste effluent drops below the minimum atomization pressure, a condition known as “fall out” may occur during which the hydrocarbon-containing waste effluent is not combusted but instead is discharged into the surrounding environment.

Some conventional test well burners propose the use of pressure responsive valves to reduce the occurrences of fall out. These valves prevent the flow of waste effluent through the burner nozzles until the waste effluent reaches a pressure sufficient to move the valves from a closed position to an open position to permit effluent flow to the nozzles. Due to variances in manufacturing, the valves may open at different effluent pressures. Should the first valves to open not be located adjacent a pilot flame, the effluent will not be combusted and fall out will occur. Additionally, when multiple nozzles are used, a control panel may be provided to add to or remove from operation one or more selected valves, thereby to provide turndown to accommodate a wide range of waste effluent flow rates. The control panel typically includes control valves that must be manually operated to activate or deactivate a particular valve or set of valves. This requires an operator to be stationed at the control panel an observe combustion during flaring operations and adjust the manual control valves so that the number of active nozzles matches the effluent flow rate.

SUMMARY OF THE DESCRIPTION

Systems and methods are disclosed herein for combusting fluids having a hydrocarbon content. The fluids may be generated during well testing, oil spill cleanup, or other operations. According to the systems and methods disclosed herein, flow control elements used to control waste effluent flow to burner nozzles are biased to a closed position by a biasing force. Reduced biasing forces are applied to selected flow control elements so that the burner nozzles associated with the selected flow control elements will open first during startup and close last during shut down. The flow control elements receiving reduced biasing forces may be selected according to proximity of the associated burner nozzles to one or more pilots so that combustion occurs more reliably and fallout of unburned waste effluent is minimized.

In accordance with certain aspects of the disclosure, a method of controlling combustion of a waste effluent containing hydrocarbons may include providing a pilot configured to generate a pilot flame, controlling waste effluent flow to a first burner nozzle through a first flow control element, the first flow control element configured to automatically move from a closed position to an open position at a first pressure threshold, and controlling waste effluent flow to a second burner nozzle through a second flow control element, the second flow control element configured to automatically move from a closed position to an open position at a second pressure threshold greater than the first pressure threshold, wherein the first burner nozzle is positioned nearer the pilot than the second burner nozzle outlet. The first flow control element is opened and the second flow control element is closed when the waste effluent flow has a waste effluent pressure above the first pressure threshold and below the second pressure threshold.

In accordance with additional aspects of the disclosure, a method of controlling combustion of a waste effluent containing hydrocarbons may include providing a pilot configured to generate a pilot flame and communicating the waste effluent to a plurality of flow control elements, the plurality of flow control elements including at least a first flow control element communicating with a first burner nozzle located nearest the pilot, a second flow control element communicating with a second burner nozzle located adjacent the first burner nozzle, and a third flow control element communicating with a third burner nozzle located farther from the first burner nozzle than the second burner nozzle. The first burner nozzle may be automatically opened when the waste effluent has a waste effluent pressure at a first waste effluent pressure threshold, thereby to permit waste effluent flow to the first burner nozzle, and the second burner nozzle may be automatically opened when the waste effluent has a waste effluent pressure at a second waste effluent pressure threshold higher than the first waste effluent pressure threshold, thereby to permit waste effluent flow to the first and second burner nozzles.

In accordance with additional aspects of the disclosure, a system for combusting waste effluent containing hydrocarbons may include a waste effluent conduit fluidly communicating with a source of waste effluent, a pilot configured to generate a pilot flame, a first burner nozzle fluidly communicating with the waste effluent conduit, and a second burner nozzle fluidly communicating with the waste effluent conduit, wherein the first burner nozzle outlet is positioned nearer the pilot than the second burner nozzle outlet. A first flow control element is disposed between the waste effluent conduit and the first burner nozzle and biased toward a closed position to block waste effluent flow to the first burner nozzle by a first biasing force. The first flow control element is configured to receive the waste effluent pressure and generate a first resulting force on the first flow control element, wherein the first resulting force counteracts the first biasing force to move the first flow control element automatically to an open position to permit waste effluent flow to the first burner nozzle when the waste effluent pressure exceeds a first waste effluent pressure threshold. A second flow control element is disposed between the waste effluent conduit and the second burner nozzle and biased toward a closed position to block waste effluent flow to the second burner nozzle by a second biasing force. The second flow control element is configured to receive the waste effluent pressure and generate a second resulting force on the second flow control element, wherein the second resulting force counteracts the second biasing force to automatically move the second flow control element to an open position to permit waste effluent to flow to the second burner nozzle when the waste effluent pressure exceeds a second waste effluent pressure threshold, and wherein the second waste effluent pressure threshold is greater than the first waste effluent pressure threshold.

The summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of methods and apparatus for combusting a multi-phase hydrocarbon fluid are described with reference to the following figures. The same numbers are used throughout the figures to reference like features and components.

FIG. 1 is a perspective view of a well test burner system constructed according to the present disclosure.

FIG. 2 is an enlarged front view of a burner head assembly used in the well test burner system of FIG. 1.

FIG. 3 is a schematic diagram of a control fluid assembly used in the well test burner system of FIG. 1.

FIG. 4 is a schematic illustration of the forces acting on an exemplary flow control element during operation of the burner system.

It should be understood that the drawings are not necessarily to scale and that the disclosed embodiments are sometimes illustrated diagrammatically and in partial views. In certain instances, details which are not necessary for an understanding of the disclosed methods and apparatuses or which render other details difficult to perceive may have been omitted. It should be understood, of course, that this disclosure is not limited to the particular embodiments illustrated herein.

DETAILED DESCRIPTION

So that the above features and advantages of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to the embodiments thereof that are illustrated in the accompanying drawings. It is to be noted, however, that the drawings illustrate only typical embodiments of this disclosure and therefore are not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments.

Methods and apparatus are disclosed herein for combusting waste effluent generated by well testing, oil spill cleanup, or other operations. The term “waste effluent” is intended to encompass any fluid having a hydrocarbon content capable of being disposed of by combustion. The waste effluent may include a liquid hydrocarbon content (such as oil), a gas hydrocarbon content (such as methane), and non-hydrocarbon containing content (such as seawater). The waste effluent may be obtained from effluent from a supply line formed during well testing operations, oil-water mixtures created during an oil spill cleanup, or other sources.

FIG. 1 illustrates an exemplary burner 20 for combusting waste effluent having a hydrocarbon content. The burner 20 includes a support 22 defining a support conduit 24. A waste effluent conduit, such as main burner pipe 26, is coupled to the support 22 so that the main burner pipe 26 fluidly communicates with the support conduit 24. Accordingly, waste effluent communicated to the support conduit 24 from a source 25 of waste effluent (FIG. 3) will pass into the main burner pipe 26.

The burner 20 further includes a burner head assembly 30 for discharging the waste effluent in a spray pattern suitable for combustion by open flame. As best shown with reference to FIGS. 1 and 2, the burner head assembly 30 is coupled to the main burner pipe 26 and includes a plurality of burner nozzles 32 oriented in an array. In the exemplary embodiment, the burner head assembly 30 includes twelve burner nozzles 32 extending radially outwardly from a center of the burner head assembly 30. In FIG. 2, each burner nozzle is designated with a unique reference numeral that implies a specific position of the burner nozzle relative to the hour positions of a clock. Accordingly, the burner nozzles are identified as 32-1, 32-2, 32-3, 32-4, 32-5, 32-6, 32-7, 32-8, 32-9, 32-10, 32-11, and 32-12. Each of the burner nozzles 32 is substantially identical and includes a burner nozzle outlet 34. The burner nozzle outlets 34 are configured to direct the waste effluent radially outwardly to form the desired combustion spray pattern.

The burner head assembly 30 also includes a pilot system for initiating flame. In the illustrated embodiment, the pilot system includes a first pilot 36 and a second pilot 38. The first and second pilots 36, 38 fluidly communicate with a pilot pipe 40. The pilot pipe 40, in turn, fluidly communicates with a pilot fuel source, which may be obtained from or independently of the waste effluent. FIG. 2 illustrates exemplary locations for the pilots, with the first pilot 36 being substantially aligned with the ninth burner nozzle 32-9 and the second pilot 38 being positioned between the fourth burner nozzle 32-4 and the fifth burner nozzle 32-5. The first and second pilots 36, 38 are configured to generate a pilot flame for igniting the waste effluent as it is discharged from the burner nozzles 32.

Each of the burner nozzles 32 includes an associated flow control element 42 for controlling flow of waste effluent through the associated nozzle 32. As shown schematically in FIG. 3, the flow control elements 42 may be provided in the form of shuttle valves, however other types of control elements may be used. In the exemplary embodiment, each flow control element 42 includes a valve body 44 that is movable between a closed position, which blocks fluid communication between the waste effluent conduit and an associated burner nozzle 32, and an open position, which permits fluid communication between the waste effluent conduit and the associated burner nozzle 32. In FIG. 3, each valve body 44 is shown in the closed position to block flow of waste effluent to the burner nozzles 32. The flow control elements may be configured to automatically move from a closed position to an open position at associated pressure thresholds. In some embodiments, the flow control elements may have different pressure thresholds so that the flow control elements open at different effluent pressures, as described in greater detail below.

FIG. 3 also schematically illustrates an exemplary control fluid assembly 50 configured to deliver control fluid at a predetermined pressure to the flow control elements 42 thereby to bias the flow control elements 42 in closed positions with biasing forces. In the exemplary embodiment, the control fluid assembly 50 includes a control fluid conduit 52 fluidly communicating with a source 54 of control fluid. The control fluid conduit 52 may include multiple branches leading to sub-sets of flow control elements 42. For example, the control fluid conduit 52 may include a first control fluid branch 56 fluidly communicating with a first sub-set of flow control elements associated with burner nozzles 32-1, 32-2, 32-3, 32-6, 32-7, 32-11, and 32-12. The control fluid conduit 52 may include a second control fluid branch 58 fluidly communicating with a second sub-set of flow control elements associated with burner nozzles 32-4 and 32-5. Additionally, the control fluid conduit 52 may include a third control fluid branch 60 fluidly communicating with a third sub-set of flow control elements associated with burner nozzles 32-8, 32-9, and 32-10.

While the exemplary embodiment includes three control fluid branches, it will be appreciated that only two or more than three control fluid branches may be provided. Furthermore, while each of the first, second, and third control fluid branches 56, 58, 60 are shown in fluid communication with multiple flow control elements 42, it will be appreciated that each flow control element 42 may have a dedicated control fluid branch. Thus, for the illustrated embodiment having twelve flow control elements 42, the control fluid conduit 52 may include twelve separate control fluid branches, each of which may be dedicated to a single associated flow control element.

The control fluid assembly 50 may also include regulators configured to provide a desired control fluid pressure in each control fluid branch. As shown in FIG. 3, a first control fluid regulator 62 is disposed in the first control fluid branch 56 to deliver control fluid to the first sub-set of flow control elements at a first control fluid pressure P1. Similarly, a second control fluid regulator 64 is disposed in the second control fluid branch 58 to deliver control fluid to the second sub-set of flow control elements at a second control fluid pressure P2. Still further, a third control fluid regulator 66 is disposed in the third control fluid branch 60 to deliver control fluid to the third sub-set of flow control elements at a third control fluid pressure P3. Each of the first, second, and third control fluid regulators 62, 64, 66 may be configured to deliver a different control fluid pressure, thereby to vary the biasing forces acting on the first, second, and third sub-sets of flow control elements. Furthermore, each regulator may be adjustable to vary the first, second, and third control fluid pressures P1, P2, P3 as desired.

The control fluid assembly 50 may further include feedback devices to provide an indication of the control fluid pressures delivered to each control fluid branch. As shown in FIG. 3, a first pressure sensor 70 is disposed in the first control fluid branch 56, a second pressure sensor 72 is disposed in the second control fluid branch 58, and a third pressure sensor 74 is disposed in the third control fluid branch 60.

Additionally, the control fluid assembly 50 may include vent valves to selectively divert control fluid pressure away from the flow control elements 42. For example, a first vent valve 80 is disposed in the first control fluid branch 56, a second vent valve 82 is disposed in the second control fluid branch 58, and a third vent valve 84 is disposed in the third control fluid branch 60. Each vent valve 80, 82, 84 may have a first position in which control fluid is permitted to flow to the flow control elements 42 and a second position which diverts control fluid to a vent line 86 leading to atmosphere. Each vent valve 80, 82, 84 may be operated manually, electrically, hydraulically, or otherwise.

Each of the components of the control fluid assembly 50 may be housed in a panel 88. Providing the control fluid assembly components in one location permits an operator to observe control fluid pressures P1, P2, P3 indicated by the pressure sensors 70, 72, 74 and adjust output pressure settings of the regulators 62, 64, 66. Additionally, the vent valves 80, 82, 84 may be manually operated from the same location, if needed.

The forces acting on a given flow control element during operation of the system are schematically illustrated at FIG. 4. In the exemplary flow control element 42, the valve body 44 is shown in the closed position, in which waste effluent flow from the main burner pipe 26 to the burner nozzle 32 is cut off. Control fluid pressure communicated from the control fluid conduit 54 creates a control fluid force, indicated by arrows 90, that biases the valve body 44 toward the closed position. Waste effluent pressure communicated from the main burner pipe 26 creates a resulting force, indicated by arrows 92, on the valve body 44 which counteracts the control fluid force. Additionally, each flow control element 42 may further include a biasing element, such as control element spring 95, configured to apply a biasing force 93 on the valve body 44 that is oriented to bias the valve body 44 toward the open position. As the waste effluent pressure increases, the resulting force 92 also increases. When the resulting force 92 and biasing force 93 exceed the control fluid force 90, the valve body 44 moves from the closed position to an open position, indicated by phantom lines in FIG. 4. In this position, waste effluent is permitted to flow to the burner nozzle 32 associated with the given flow control element 42.

In operation, a user may select different control fluid pressures P1, P2, P3 so that flow control elements 42 will open in a predetermined sequence. In the illustrated embodiment, for example, P2 and P3 may be set lower than P1, so that the burner nozzles 32-8, 32-9, and 32-10 located nearer the first pilot 36 and the burner nozzles 32-4 and 32-5 located nearer the second pilot 38 open before the remaining burner nozzles 32-1, 32-2, 32-3, 32-6, 32-7, 32-10, 32-11, and 32-12. Accordingly, when the pilot 36 is ignited to produce a pilot flame, only the nozzles nearest the pilots 36, 38 will initially discharge effluent so that combustion is more reliably obtained and fallout is decreased. Subsequently, as the effluent pressure increases, effluent discharged from the remaining nozzles will be ignited by combustion present at the initial nozzles.

In view of the foregoing, systems and methods are provided for combusting waste effluent having a hydrocarbon content. The systems and methods employ flow control elements leading to burner nozzles that are biased toward a closed position by a biasing force. Selected flow control elements receive a reduced biasing force so that they are the first to open in response to an increasing waste effluent pressure during startup, and are the last to close in response to a decreasing waste effluent pressure during shutdown. In some embodiments, the biasing force is provided by a control fluid system that communicates control fluid to each of the flow control elements. Regulators may be provided to reduce the control fluid pressure to selected flow control elements, thereby to provide the different biasing forces. Waste effluent pressure is also communicated to the flow control elements to generate a resulting force, so that each flow control element will automatically move to an open position when the resulting force is greater than the biasing force. The flow control elements that receive reduced biasing forces may be selected based on the proximity of the burner nozzles associated with those selected flow control elements to one or more pilots. Specifically, burner nozzles located nearer the pilots may have the lowest biasing force so that they are the first to open, with gradually increasing biasing forces being applied to the flow control elements of nozzles located farther away from the pilots. By first opening the burner nozzles located nearest the pilots, combustion of waste effluent is more reliably obtained and fallout of unburned waste effluent is decreased.

Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from the burner assembly and methods for flaring low calorific content gases disclosed and claimed herein. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. §112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function. 

What is claimed is:
 1. A method of controlling combustion of a waste effluent containing hydrocarbons, the method comprising: providing a pilot configured to generate a pilot flame; controlling waste effluent flow to a first burner nozzle through a first flow control element, the first flow control element configured to automatically move from a closed position to an open position at a first pressure threshold; controlling waste effluent flow to a second burner nozzle through a second flow control element, the second flow control element configured to automatically move from a closed position to an open position at a second pressure threshold greater than the first pressure threshold, wherein the first burner nozzle is positioned nearer the pilot than the second burner nozzle outlet; and opening the first flow control element and closing the second flow control element when the waste effluent flow has a waste effluent pressure above the first pressure threshold and below the second pressure threshold.
 2. The method of claim 1, further comprising: biasing the first flow control element with a first biasing force toward the closed position to block waste effluent flow to the first burner nozzle; and biasing the second flow control element with a second biasing force toward the closed position to block waste effluent flow to the second burner nozzle, wherein the second biasing force is greater than the first biasing force.
 3. The method of claim 2, further comprising: communicating control fluid at a first control fluid pressure to the first flow control element to provide the first biasing force; and communicating control fluid at a second control fluid pressure to the second flow control element to provide the second biasing force.
 4. The method of claim 3, in which a first control fluid regulator provides the first control fluid pressure and a second control fluid regulator provides the second control fluid pressure.
 5. The method of claim 1, further comprising: controlling waste effluent flow to a third burner nozzle through a third flow control element, the third flow control element configured to automatically move from a closed position to an open position at a third pressure threshold greater than the first and second pressure thresholds, the third burner nozzle having a third burner nozzle outlet, wherein the first and second burner nozzle outlets are positioned nearer the first pilot than the third burner nozzle outlet; and opening the first and second flow control elements and closing the third flow control element when the waste effluent flow has a waste effluent pressure above the second pressure threshold and below the third pressure threshold.
 6. The method of claim 1, further comprising initiating the pilot flame to ignite waste effluent flowing from the first burner nozzle.
 7. A method of controlling combustion of a waste effluent containing hydrocarbons, the method comprising: providing a pilot configured to generate a pilot flame; communicating the waste effluent to a plurality of flow control elements, the plurality of flow control elements including at least a first flow control element communicating with a first burner nozzle located nearest the pilot, a second flow control element communicating with a second burner nozzle located adjacent the first burner nozzle, and a third flow control element communicating with a third burner nozzle located farther from the first burner nozzle than the second burner nozzle; automatically opening the first burner nozzle when the waste effluent has a waste effluent pressure at a first waste effluent pressure threshold, thereby to permit waste effluent flow to the first burner nozzle; and automatically opening the second burner nozzle when the waste effluent has a waste effluent pressure at a second waste effluent pressure threshold higher than the first waste effluent pressure threshold, thereby to permit waste effluent flow to the first and second burner nozzles.
 8. The method of claim 7, further comprising: biasing the first flow control element with a first biasing force toward a closed position to block waste effluent flow to the first burner nozzle; biasing the second flow control element with a second biasing force toward a closed position to block waste effluent flow to the second burner nozzle, wherein the first biasing force is greater than the second biasing force; and biasing the third flow control element with a third biasing force toward a closed position to block waste effluent flow to the third burner nozzle, wherein the second biasing force is greater than the third biasing force; wherein the first waste effluent pressure threshold is substantially equal to the first biasing force and the second waste effluent pressure threshold is substantially equal to the second biasing force.
 9. The method of claim 8, further comprising: communicating control fluid at a first control fluid pressure to the first flow control element to provide the first biasing force; communicating control fluid at a second control fluid pressure to the second flow control element to provide the second biasing force; and communicating control fluid at a third control fluid pressure to the third flow control element to provide the third biasing force.
 10. The method of claim 9, in which a first control fluid regulator provides the first control fluid pressure, a second control fluid regulator provides the second control fluid pressure, and a third control fluid regulator provides the third control fluid pressure.
 11. The method of claim 7, further comprising initiating the pilot flame to ignite combustion of the waste effluent flowing from the first burner nozzle when the waste effluent has a waste effluent pressure at the first waste effluent pressure threshold.
 12. The method of claim 11, in which combustion of the waste effluent flowing from the first burner nozzle ignites combustion of the waste effluent flowing from the second burner nozzle when the waste effluent has a waste effluent pressure at the second waste effluent pressure threshold.
 13. A system for combusting waste effluent containing hydrocarbons, the system comprising: a waste effluent conduit fluidly communicating with a source of waste effluent; a pilot configured to generate a pilot flame; a first burner nozzle fluidly communicating with the waste effluent conduit; a second burner nozzle fluidly communicating with the waste effluent conduit, wherein the first burner nozzle outlet is positioned nearer the pilot than the second burner nozzle outlet; a first flow control element disposed between the waste effluent conduit and the first burner nozzle and biased toward a closed position to block waste effluent flow to the first burner nozzle by a first biasing force, the first flow control element configured to receive the waste effluent pressure and generate a first resulting force on the first flow control element, wherein the first resulting force counteracts the first biasing force to move the first flow control element automatically to an open position to permit waste effluent flow to the first burner nozzle when the waste effluent pressure exceeds a first waste effluent pressure threshold; and a second flow control element disposed between the waste effluent conduit and the second burner nozzle and biased toward a closed position to block waste effluent flow to the second burner nozzle by a second biasing force, the second flow control element configured to receive the waste effluent pressure and generate a second resulting force on the second flow control element, wherein the second resulting force counteracts the second biasing force to automatically move the second flow control element to an open position to permit waste effluent to flow to the second burner nozzle when the waste effluent pressure exceeds a second waste effluent pressure threshold, and wherein the second waste effluent pressure threshold is greater than the first waste effluent pressure threshold.
 14. The system of claim 13, further comprising: a third burner nozzle fluidly communicating with the waste effluent conduit and located farther from the pilot than the second burner nozzle; and a third flow control element disposed between the waste effluent conduit and the third burner nozzle and biased toward a closed position to block waste effluent flow to the third burner nozzle by a third biasing force, the third flow control element configured to receive the waste effluent pressure and generate a third resulting force on the third flow control element, wherein the third resulting force counteracts the third biasing force to automatically move the third flow control element to an open position to permit waste effluent to flow to the third burner nozzle when the waste effluent pressure exceeds a third waste effluent pressure threshold, and wherein the third waste effluent pressure threshold is greater than the first waste effluent pressure threshold.
 15. The system of claim 14, in which the third waste effluent pressure threshold is greater than the second waste effluent pressure threshold. 